Method and device for self-clock controlled pseudo random noise (PN) sequence generation

ABSTRACT

This invention relates to an electrical device for self-clocked controlled pseudo random noise (PN) sequence generation and comprising a plurality of sequence generation means adapted to:  
     output a plurality of sequence values (Z t ) on the basis of a plurality of clock values (C t ),  
     wherein said electrical device further comprises:  
     step pattern generation means ( 202 ) adapted to select a step pattern, comprising said plurality of clock values (C t ), from a plurality of possible step patterns on the basis of a step pattern select signal (W t ).  
     Hereby, a flexible and efficient self-clocked controlled pseudo random noise (PN) sequence generation is obtained.  
     This invention also relates to a method of self-clocked controlled pseudo random noise (PN) sequence generation.

[0001] The present invention relates to an electrical device forself-clocked controlled pseudo random noise (PN) sequence generation andcomprising a plurality of sequence generation means adapted to:

[0002] output a plurality of sequence values (Z_(t)) on the basis of aplurality of clock values (C_(t)).

[0003] The present invention also relates to a method of self-clockcontrolled pseudo random noise (PN) sequence generation comprising thestep of:

[0004] outputting a plurality of sequence values (Z_(t)) on the basis ofa plurality of clock values (C_(t)).

[0005] Pseudo random noise sequences (PN sequences) are used in manycryptographic and communication applications to provide randomlyappearing symbols. Typically, cryptographic applications are methods toprovide confidentiality of transmitted information through the use ofstream ciphers. In communications systems PN sequences may e.g. be usedas spreading sequences in spread-spectrum communications systems wherethey determine the hop sequence and/or the direct spreading sequence.

[0006] In general, a receiver of a spread-spectrum communications systemwill receive a digital signal/bit stream transmitted over a singlecarrier frequency which is combined from a digital signal/bit streamcontaining information such as a digitized voice and from a PN sequenceused to code or encrypt the transmission. Typically, the length of thePN sequence stream is much larger than the length of the informationstream.

[0007] The PN sequences are sometimes derived by using a maximal lengthpolynomial. Constructions, whether hardware or software implemented,which form PN sequences in this manner are sometimes referred to asm-sequence generators. It is well known that the randomness propertiesof the sequences generated by the m-sequence generators are very limitedas a result of linear relationships between the symbols of the sequence.This enables a prediction of the next symbol given sufficiently many,but small number of previous symbols. In various applications this isnot desirable and, hence, there is a need for efficient techniques toenhance the unpredictability.

[0008] Clock control of the m-sequence generator is a well-known methodthat can be used to increase the unpredictability of m-sequencegenerators.

[0009] In most cryptographic applications the outputs of several suchclock controlled m-sequence generators are combined to generate thefinal output sequence. Output bits generated by a combination of clockcontrolled m-sequence generators form the PN sequence which are used toe.g. encrypt or spread an information signal.

[0010] In known applications, e.g. in the ciphering generator A5/1 ofthe GSM cellular mobile telephone system, the clocking signals forcontrolling the stepping of the involved clocked m-sequence generatorsare derived from signals of the clocked m-sequence generatorsthemselves, i.e. the generators are self-clocked.

[0011] The A5/1 generator will be described in more detail in connectionwith FIG. 1, which shows a schematic block diagram of the A5/1generator.

[0012] In the A5/1 generator, three signals (101) are taken from threefeedback registers (104) and combined by mapping means (102) in such away as to derive three clock control signals (103) so that at least twofeedback shift registers (104) and sometimes three are always stepped.

[0013] There are eight possible values for the combination of inputsignals (101) which are mapped to the four possible stepping patterns(0,1,1), (1,0,1), (1,1,0), and (1,1,1) (103) if at least two registersare to be stepped. Here (x,y,z) means that if x is 0, the first feedbackregister is not stepped, and if x is 1, the first feedback register isstepped. In the same way, y and z control the second and third feedbackregisters, respectively.

[0014] Since each feedback shift register (104) is stepped at leastthree times out of the four possible stepping patterns (102), each shiftregister (104) is stepped six times out of the eight possiblecombinations of the input signals (101), since these eight combinationsare mapped to the four stepping patterns (103).

[0015] Such a uniformity of stepping each shift register (104),responsible for clocking/controlling the connected m-sequence generators(not shown), has an enhancing effect on the unpredictability of thefinal output PN sequence combined from the output of the m-sequencegenerators.

[0016] However, it is very hard in general to realise such a favourableclocking mechanism with presently known techniques, as e.g. used inA5/1, for other self-clocking methods and/or schemes; especially formethods using a higher number of shift registers (and thereby morem-sequence generators), which is needed e.g. for a higher degree ofunpredictability.

[0017] Another problem with the prior art as described above is if e.g.four m-sequence generators (and thereby four shift registers) are needede.g. for a higher degree of unpredictability, and only two shiftregisters are to be stepped at a time, 16 combinations of input valuesmust be mapped to the 6 possible step patterns (0,0,1,1), (0,1,0,1),(1,0,0,1), (0,1,1,0), (1,0,1,0), (1,1,0,0). With presently knowntechniques some step patterns will be used more often since the 16combinations are mapped to only 6 step patterns.

[0018] Further U.S. Pat. No. 4,817,145 discloses a generator forgenerating pseudo random binary cipher sequences comprising a number ofsubgenerators arranged in a cascaded scheme. A first subgeneratorclocks/steps the other subgenerators and is always clocked itself by anexternal clock at a constant rate.

[0019] Even though this prior generator enhances the randomness of theoutput sequence it is relatively easier to cryptoanalyse since one partof the generator behaves regularly.

[0020] An object of the invention is to provide an electrical device forself clocked controlled PN sequence generation where a plurality of PNsequence generators are clocked uniformly with respect to thecombinations of input signals.

[0021] Another object is to provide flexibility with respect toimplementing clock control, so that more freedom with respect to steppatterns is possible/available.

[0022] This object is achieved by an electrical device of theaforementioned type further comprising:

[0023] step pattern generation means adapted to select a step pattern,providing said plurality of clock values (C_(t)), from a plurality ofpossible step patterns on the basis of a step pattern select signal(W_(t)).

[0024] Hereby, a final output PN sequence generation will have anenhanced degree of unpredictability since the generation means areclocked uniformly.

[0025] Additionally, the uniformity is obtained on the basis of a steppattern select signal (W_(t)) without using shift registers, whichenhances the flexibility of the device, since the step pattern selectsignal (W_(t)), responsible for controlling/clocking the sequencegeneration means, may be obtained by various combinations of differentsignals instead of mapping means in combination with shift registers.

[0026] The enhanced unpredictability and uniformity may be obtained forany number of sequence generation means and any number of steppingpatterns.

[0027] If, as stated in claim 2, said step pattern select signal (W_(t))is derived on the basis of a combined value (U_(t)) and one or morepreviously derived step pattern select signals (W_(t-1)), a very simpleand unpredictable way of deriving the step pattern select signal (W_(t))is obtained.

[0028] Additionally, the step pattern select signal (W_(t)) may beprovided by using little additional hardware.

[0029] In an alternative embodiment, as stated in claim 3, saidplurality of sequence generation means (201) is further adapted tooutput a plurality of step control values (u_(t)), and said combinedvalue (U_(t)) is provided on the basis of said plurality of step controlvalues (u_(t)) and on the basis of a plurality of prior clock values(C_(t-1)).

[0030] Hereby, an efficient and simple use of existing signals isobtained.

[0031] A simple way of calculating a step pattern select signal (W_(t))is obtained, if, as stated in claim 4, the number of said plurality ofpossible step patterns is 6, and said pattern select signal (W_(t)) isderived as: U_(t)+W_(t-1) MOD 6.

[0032] In this way a very simple way of calculating the step patternselect signal (W_(t)) is obtained by using existing signals, while stillobtaining uniformity with respect to the clocking of the sequencegeneration means.

[0033] For another number of possible step patterns than 6 other modulofunctions may be applied, e.g. MOD 4 for four possible step patterns.

[0034] Alternatively, as stated in claim 5, if the number of saidplurality of possible step patterns is 6, and said pattern select signal(W_(t)) is derived as: U_(t)+a₁W_(t-1)+a₂W_(t-2)+a₃W_(t-3) MOD 6, wherea₁, a₂, and a₃ are pre-selected constants, an even better, i.e.resulting in a more unpredictable output PN sequence, way of computingthe pattern select signal (W_(t)) is obtained.

[0035] Alternatively, as stated in claim 6, if the number of saidplurality of possible step patterns is not a prime number, then saidpattern select signal (W_(t)) is derived on the basis of said combinedvalue (U_(t)) and said previously derived step pattern select signals(W_(t-1)) using a Chinese remaindering technique.

[0036] In this way a very efficient calculation of the pattern selectsignal (W_(t)) is obtained, since some calculations of the MOD functionmay be executed simultaneously.

[0037] Another object of the invention is to provide an electricaldevice comprising a plurality of stepping patterns which reduces powerconsumption.

[0038] This is obtained, as stated in claim 7, by choosing saidplurality of possible step patterns to be: (0,0,1,1), (0,1,0,1),(1,0,0,1), (0,1,1,0), (1,0,1,0), (1,1,0,0).

[0039] In this way only two generators are clocked at a time, therebyreducing the power needed.

[0040] Additionally, the total power consumption of the total electricaldevice is independent of the specific stepping pattern used.

[0041] In a preferred embodiment, as stated in claim 8, said devicefurther comprises function generating means (203) adapted to calculatean output value (Out_(t)) as the sum of said plurality of sequencevalues (Z_(t)) MOD 2.

[0042] In a preferred embodiment, as stated in claim 9, said pluralityof sequence generation means is m-sequence generators.

[0043] A further object of the invention is to provide a method ofself-clocked controlled PN sequence generation where the PN sequencegenerators are clocked uniformly with respect to the combinations ofinput signals.

[0044] Another object is to provide flexibility with respect toimplementing clock control, so that more freedom with respect to steppatterns is possible/available.

[0045] This object is achieved by said method of the aforementioned typefurther comprising the step of:

[0046] selecting a step pattern, providing said plurality of clockvalues (C_(t)), from a plurality of possible step patterns on the basisof a step pattern select signal (W_(t)).

[0047] Hereby, a final output PN sequence generation will have anenhanced degree of unpredictability since the generationmeans/generators are clocked uniformly.

[0048] Additionally, the uniformity is obtained on the basis of a steppattern select signal (W_(t)) without using shift registers, whichenhances the flexibility of the method, since the step pattern selectsignal (W_(t)) may be obtained in many different ways by variouscombinations of different signals and/or values instead of mapping meansin combination with shift registers.

[0049] In a preferred embodiment, the step pattern select signal (W_(t))is derived on the basis of a combined value (U_(t)) and one or morepreviously derived step pattern select signals (W_(t-1)).

[0050] Hereby, a very simple way of deriving the step pattern selectsignal (W_(t)) is obtained.

[0051] Additionally, the step pattern select signal (W_(t)) may becalculated by using little computational time/complexity.

[0052] In yet another embodiment, a plurality of step control values(u_(t)) is output, and said combined value (U_(t)) is provided on thebasis of said plurality of step control values (u_(t)) and on the basisof a plurality of prior clock values (C_(t-1)).

[0053] Hereby, an efficient and simple use of existing values isobtained.

[0054] In one embodiment, the number of said plurality of possible steppatterns is 6, and said pattern select signal (W_(t)) is derived as:U_(t)+W_(t-1) MOD 6.

[0055] In another embodiment, the number of said plurality of possiblestep patterns is 6, and said pattern select signal (W_(t)) is derivedas: U_(t)+a₁W_(t-1)+a₂W_(t-2)+a₃W_(t-3) MOD 6, where a₁, a₂, and a₃ arepre-selected constants.

[0056] In yet another embodiment, said pattern select signal (W_(t)) isderived on the basis of said combined value (U_(t)) and said previouslyderived step pattern select signals (W_(t-1)) using a Chineseremaindering technique, if the number of said plurality of possible steppatterns is not a prime number.

[0057] Another object of the invention is to provide a method comprisinga stepping pattern which reduces computational complexity even further.

[0058] This is obtained, as stated in claim 17, when said plurality ofpossible step patterns is: (0,0,1,1), (0,1,0,1), (1,0,0,1), (0,1,1,0),(1,0,1,0), (1,1,0,0).

[0059] In one embodiment, said method further comprises the step ofcalculating a value (Out_(t)) as the sum of said plurality of sequencevalues (Z_(t)) MOD 2.

[0060] In another embodiment, said plurality of sequence values (Z_(t))is generated by a plurality of m-sequence generators, e.g. implementedin software.

[0061] The present invention also relates to the use of the methodand/or electrical device mentioned above in a portable device. In apreferred embodiment the portable device is a mobile telephone.

[0062] Hereby, efficient and more safe encryption of digitized speech orother kind of digital information may be obtained.

[0063] Additionally, since only two sequence generation means areclocked at a time, electrical power is saved which is especiallyimportant in e.g. a mobile telephone.

[0064] The present invention will now be described more fully withreference to the drawings, in which

[0065]FIG. 1 shows a schematical block diagram of a prior art generator;

[0066]FIG. 2 shows four clock controlled PN sequence generatorsgenerating an output PN sequence according to the invention;

[0067]FIG. 3 shows a step pattern subsystem according to one embodimentof the invention;

[0068]FIG. 4 shows a generalized embodiment of clock controlled PNsequence generators;

[0069]FIG. 5 shows a generalized embodiment of the step patternsubsystem;

[0070]FIG. 6 shows a flow chart of the method according to theinvention;

[0071]FIG. 7 shows the preferred embodiment of the invention, which maycontain the electrical device and/or use the method according to thepresent invention;

[0072]FIGS. 8a and 8 b show two exemplary implementations of a systemusing the method and/or device according to the invention.

[0073] In the following m-sequence generators are described as anexample of a PN sequence generator, but other types of PN sequencegenerators might be just as applicable.

[0074]FIG. 1 shows a schematic block diagram of a prior art generator.This figure has already been explained above.

[0075]FIG. 2 shows four clock controlled PN sequencegenerators/generation means (201) generating an output PN sequenceaccording to the invention. Preferably, the sequence generation means(201) are m-sequence generators.

[0076] The clocking of each generator (201) is controlled by a sequenceC^(k)=C^(k) ₀, C^(k) ₁, C^(k) ₂, . . . , of symbols each representingthe value 0 or 1, i.e. C^(k) _(t) ε {0, 1} where t denotes the timeinstants 0, 1, 2, . . . and k denotes the index of the generators (201),i.e. 1, 2, 3, and 4 in this example. The sequence C^(k) _(t) is denotedclock signal which consists of clock values forming the sequence.

[0077] If the k'th m-sequence generator (201) steps once every timeinstant, the generator (201) will produce the simple sequenceX^(k)=X^(k) ₀, X^(k) ₁, X^(k) ₂, X^(k) ₃, . . . . If the stepping iscontrolled by the values of the symbols of C^(k), the following outputsequence Z^(k)=Z^(k) ₀, Z^(k) ₁, Z^(k) ₂, Z^(k) ₃, . . . , will beproduced:

Z ^(k) _(t) =X ^(k) _(σ)(k,t) t=0, 1, 2, 3, . . . ,

[0078] where

σ(k,t)=Σ_(i) C ^(k) _(i) C ^(k) _(i)ε{1,2},

[0079] and the sum Σ goes from i=0 to i=t−1 and k=1, 2, 3, 4. In otherwords, the next symbol Z^(k) _(j) is equal to either the next symbolX^(k) _(m) (if C^(k) _(t)=1) or the next symbol again X^(k) _(m+1) (ifC^(k) _(t)=2). As an example, the sequence Z^(k) ₀=X^(k) ₀, Z^(k)₁=X^(k) ₂, Z^(k) ₂=X^(k) ₄, Z^(k) ₃=X^(k) ₅ will be output if C^(k) ₀=1or 2 (does not matter), C^(k) ₁=2, C^(k) ₂=2, C^(k) ₃=1.

[0080] The four output sequences z¹, z², z³, z⁴ are combined by functiongeneration means (203) which applies a given function F to the sequencesz¹, z², z³, z⁴ in order to obtain one single output sequence Out=Out₀,Out₁, Out₂, Out₃, . . . .

[0081] As an example, F may be:

[0082] Hereby, one single PN sequence is obtained and used as outputsequence arising from a combination of the four single PN sequences,thereby enhancing the unpredictability even further.

[0083] Alternatively, other kinds of functions may be applied instead ofthe above one, e.g. a simple way of selecting one of the single PNsequences.

[0084] The clocking signals C¹ _(t), C² _(t), C³ _(t), C⁴ _(t),responsible for controlling the generators (201), form the step pattern(C¹ _(t), C² _(t), C³ _(t), C⁴ _(t)) for each time instant. The clockingsignals are derived by a step pattern subsystem (202) which receives astep control signal (u^(k) _(t)) from each generator (201).

[0085] The step control signal (u^(k) _(t)) preferably depends on theclocking signal (C^(k) _(t)).

[0086] The step pattern subsystem/step pattern generation means (202)will be described in detail in connection with FIG. 3.

[0087] The number of generators that may be used in connection with thepresent invention may be any number, and four just serves as an example.

[0088]FIG. 3 shows a step pattern subsystem (202) according to oneembodiment of the invention. The implementation shown in FIG. 3 is onlyone possible implementation of the step pattern subsystem (202).

[0089] The subsystem (202), also denoted step pattern generation means,receives four step control values u¹ _(t), u² _(t), u³ _(t), u⁴ _(t) ateach time instant. The step control values together with the clockingsignals C¹ _(t), C² _(t), C³ _(t), C⁴ _(t) are used to obtain a singlecombined value U_(t) by the combination means (302). The combined value(U_(t)) denotes a value in the range 0, 1, 2, 3.

[0090] Preferably, the following 6 possible stepping patterns arechosen: (0,0,1,1), (0,1,0,1), (1,0,0,1), (0,1,1,0), (1,0,1,0),(1,1,0,0).

[0091] These 6 stepping patterns have the advantages of stepping thefour connected generators (201) uniformly, thereby enhancing theunpredictability, since no single generator (201) is stepped more oftenthan others on average.

[0092] Another advantage is that only two generators (201) are steppedat the same time, thereby reducing the power consumption in theelectrical device, which is an advantage especially in portable deviceslike mobile telephones which have a limited power source.

[0093] These 6 possible stepping patterns are just an example and anynumber or combination is applicable.

[0094] Preferably, U_(t) is derived in the following way dependent onthe value of the previous C_(t): C_(t) U_(t) (0,0,1,1) 2 u_(t) ⁴ + u_(t)³ (0,1,0,1) 2 u_(t) ⁴ + u_(t) ² (1,0,0,1) 2 u_(t) ⁴ + u_(t) ¹ (0,1,1,0)2 u_(t) ³ + u_(t) ² (1,0,1,0) 2 u_(t) ³ + u_(t) ¹ (1,1,0,0) 2 u_(t) ² +u_(t) ¹

[0095] Any kind of other relationships may be used to derive thecombined value (U_(t)).

[0096] A step pattern select signal (W_(t)), responsible for selectingthe step pattern being forwarded to the generators (201) in patternselect means (301), is derived based on the combined value (U_(t)) and aprevious generated step pattern select signal (W_(t-1)). The previousgenerated step pattern select signal (W_(t-1)) is held in a delayelement (304).

[0097] The derivation of the step pattern select signal (W_(t)) is donein the ADD/MOD means (303), receiving the combined value (U_(t)) and aprevious generated step pattern select signal (W_(t-1)), in thefollowing way: W_(t-1)=U_(t)+W_(t-1) MOD 6.

[0098] The step pattern select signal (W_(t)) takes a value in the range0, 1, 2, 3, 4, 5.

[0099] In an alternative embodiment, the step pattern select signal(W_(t)) is derived on the basis of the combined value (U_(t)) and aplurality of previously generated step pattern select signal W_(t-1),W_(t-2), W_(t-3), . . . , giving even better properties with respect tounpredictability.

[0100] For example, the step pattern select signal (W_(t)) may bederived as: U_(t)+a₁W_(t-1)+a₂W_(t-2)+a₃W_(t-3) MOD 6, where a₁, a₂, anda₃ are pre-selected constants suitable for a given application.

[0101] In cases where the number of selectable patterns is not a primenumber, the computation of the step pattern select signal (W_(t)) mayadvantageously be implemented using a Chinese remaindering technique.For the example of 6 possible stepping patterns, the step pattern selectsignal (W_(t)) can be derived by simultaneously computing:

W ¹ _(t) =U _(t) +W ¹ _(t-1) MOD 2, t=0, 1, 2, . . .

W ² _(t) =U _(t) +W ² _(t-1) MOD 3, t=0, 1, 2, . . .

[0102] where W² _(t-1)=W_(t-1) MOD 2 and W¹ _(t-1)=W_(t-1) MOD 3, byusing 6 prime number factors (6=2×3).

[0103]FIG. 4 shows a generalized embodiment of clock controlled PNsequence generators. Shown is a plurality of N pn-sequence generators(201) and a step pattern subsystem (402), which may in functionalitycorrespond to the step pattern system (202) shown in FIG. 2, but adaptedto obtain N step control signal (u_(t) ¹ . . . u_(t) ^(N)) from the Npn-sequence generators (201).

[0104] The step pattern subsystem (402) provides the N pn-sequencegenerators (201) with N clocking signals (C_(t) ¹ . . . C_(t) ^(N))responsible for clocking/controlling the pn-sequence generators (201) byapplying a functionality e.g. similar to the one explained in connectionwith FIG. 3.

[0105] The generalized step pattern subsystem (402) will be described inconnection with FIG. 5.

[0106] Also shown is function generation means (403) which applies agiven function F to the sequences Z¹, Z², . . . Z^(N) in order to obtainone single output sequence Out=Out₀, Out₁, Out₂, Out₃, . . . , e.g. asdescribed in connection with FIG. 2.

[0107]FIG. 5 shows a generalized embodiment of the step patternsubsystem. Shown is a step pattern subsystem corresponding to the system(402) in FIG. 4 which selects one step pattern comprising N clockingsignals C_(t) among a plurality of possible step patterns.

[0108] The step pattern subsystem receives N step control values u¹_(t), u² _(t), . . . , u^(N) _(t) at each time instant from Npn-sequence generators (401), where u^(i) _(t) is 0 or 1 in the simplestcase for i=1 . . . N. The step control values are used together with aprevious step pattern/clocking signals C¹ _(t), C² _(t), . . . , C^(N)_(t) from pattern select means (501) by combination means (502) in orderto obtain a 35 single combined value U_(t).

[0109] A step pattern select signal (W_(t)), responsible for selectingthe step pattern being forwarded to the generators (401) by the patternselect means (501), is derived based on the combined value (U_(t)) and aprevious generated step pattern select signal (W_(t-1)). The previousgenerated step pattern select signal (W_(t-1)) is held in a delayelement (304).

[0110] The derivation of the step pattern select signal (W_(t)) is donein the combine input means (503), receiving the combined value (U_(t))and a previous generated step pattern select signal (W_(t-1)). Thederivation of the step pattern select signal (W_(t)) may be realized inmany ways depending of a particular implementation of the invention andmay e.g. be derived as an addition MOD M, where M is the number ofpossible step patterns, as described in connection with FIG. 3 (whereM=6).

[0111] The M step patterns may be chosen as suitable clock patterns ofall the possible combinations of subsets containing k elements from aset of N elements. A subset refers to the k pn-sequences generators tobe clocked.

[0112] In general, for a given k all the values between K_(min) andK_(max), with N≧K_(max)≧K_(min)≧1, may be chosen, giving $\begin{matrix}{M = {\sum\limits_{k = K_{\min}}^{k = K_{\max}}\frac{N!}{{k!}{\left( {N - k} \right)!}}}} & 30\end{matrix}$

[0113] For the embodiments in FIG. 2 and 3 the values was: N=4, k=2,K_(max)=K_(min)=2.

[0114] As mentioned, the embodiments shown in FIG. 2 and 3 is aspecialized case of the embodiments shown in FIGS. 4 and 5 and manyother embodiments may just be as applicable depending on a specificimplementation and should not limit the scope of the invention as soughtout in the appended claims.

[0115]FIG. 6 shows a flow chart of the method according to theinvention. The method generates a plurality of PN sequencevalues/symbols and selects one of these as output. As an example, amethod using four generators will be described.

[0116] The method is initialised at step (601).

[0117] At step (602) one step pattern is selected on the basis of a steppattern select signal (W_(t)) from a plurality of possible steppatterns.

[0118] The generation of the step pattern select signal (W_(t)) will bedescribed later in connection with step (606).

[0119] The step pattern chosen is responsible for controlling the modeaccording to which the PN sequence generators will generate a outputsequence value (Z_(t)), as will be described later.

[0120] Preferably, the following 6 possible stepping patterns arechosen: (0,0,1,1), (0,1,0,1), (1,0,0,1), (0,1,1,0), (1,0,1,0),(1,1,0,0).

[0121] These 6 possible stepping patterns have the advantage that onlytwo generators is clocked at a time, thereby reducing the computationalcomplexity of the method at a given time instance.

[0122] Another advantage is that the 6 possible stepping patterns clockthe generators uniformly, thereby enhancing the unpredictability, sinceno generator is used to output a sequence value more often than theothers.

[0123] At step (603) a plurality of clock values (C_(t)) is provided tothe generators on the basis of the selected step pattern. The steppattern comprises the individual clock values for each generator, e.g.the step pattern (0,0,1,1) will control the two first generators to steponce (generate the next symbol) and control the two last generators tostep twice (generate the second next symbol).

[0124] After receiving the clock values (C_(t)), the generators eachgenerate a PN sequence value according to the received clock value C_(t)at step (604).

[0125] At step (605) a function F is applied to combine the four outputsequences Z¹, Z², Z³, Z⁴ in order to obtain one single output sequenceOut=Out₀, Out₁, Out₂, Out₃, . . . .

[0126] Preferably, F is defined as:

[0127] where Z^(k) _(t) denotes a sequence value (Z_(t)) generated bythe k'th generator.

[0128] In this way, a single sequence value (Out_(t)) is obtained whichrelates to all the generated sequence values (Z_(t)), thereby enhancingthe unpredictability of the output sequence (Out_(t)) even further.

[0129] Other functions than the one described above may be applied justas well.

[0130] At step (606) a plurality of step control values (u_(t)) isprovided from the plurality of generators. These step control values(u_(t)) together with the clock values (C_(t)) are used to derive acombined value (U_(t)) which is used to calculate the step patternselect signal (W_(t)) as described in connection with the deviceaccording to the invention.

[0131] Preferably, the generators are standard m-sequence generators,e.g. implemented in software, but other types of PN sequence generatorsmay be used instead.

[0132] Alternatively, some of the steps may be executed simultaneously,e.g. the steps (603) and (606).

[0133]FIG. 7 shows a preferred embodiment of the invention, which maycontain the electrical device and/or use the method according to thepresent invention. Shown is a mobile telephone (701) having displaymeans (704), a keypad (705), an antenna (702), a microphone (706), and aspeaker (703). By including the electrical device and/or the methodaccording to the present invention a safer encryption of speech signalis provided, just requiring very little additional hardware and/oradditional computational effort.

[0134]FIG. 8a shows a communications system (801) comprising a firsttransmitting/receiving station (803) and a second transmitting/receivingstation (804) where information (805) may be transmitted. The PNsequences generated by the device or method according to embodiments ofthe present invention may be used as a subcomponent to encryptinformation (805) to be transmitted between the firsttransmitting/receiving station (803) and the secondtransmitting/receiving station (804).

[0135] In this way, a safer transmission of information (805), likedata, digitized speech signals, etc., may be achieved with reduced powerconsumption and/or reduced computational complexity.

[0136]FIG. 7b shows a transmitting/receiving station (803) and a mobileterminal (701) which form a cellular communications system (802). Theinformation (805) to be transmitted/received between the mobile terminal(701) and a network infrastructure (not shown) via thetransmitting/receiving station (803) may be encrypted through the use ofa ciphering system that uses PN sequences generated by clock controlledm-sequence generators.

[0137] In this way, safe transmission of information (705), like data,digitized speech signals, etc., may be achieved by using less hardware,thereby reducing the costs and power consumption.

1. An electrical device for self-clocked controlled pseudo random noise(PN) sequence generation and comprising a plurality of sequencegeneration means (201) adapted to: output a plurality of sequence values(Z_(t)) on the basis of a plurality of clock values (C_(t)),characterized in that said electrical device further comprises: steppattern generation means (202) adapted to select a step pattern,comprising said plurality of clock values (C_(t)), from a plurality ofpossible step patterns on the basis of a step pattern select signal(W_(t)).
 2. An electrical device according to claim 1, characterized inthat said step pattern select signal (W_(t)) is derived on the basis ofa combined value (U_(t)) and one or more previously derived step patternselect signals (W_(t-1)).
 3. An electrical device according to claim 2,characterized in that said plurality of sequence generation means (201)is further adapted to output a plurality of step control values (u_(t)),and said combined value (U_(t)) is provided on the basis of saidplurality of step control values (u_(t)) and on the basis of a pluralityof prior clock values (C_(t-1)).
 4. An electrical device according toclaim 2 or 3, characterized in that the number of said plurality ofpossible step patterns is 6, and in that said pattern select signal(W_(t)) is derived as: U_(t)+W_(t-1) MOD
 6. 5. An electrical deviceaccording to claim 2 or 3, characterized in that the number of saidplurality of possible step patterns is 6, and in that said patternselect signal (W_(t)) is derived as: U_(t)+a₁W_(t-1)+a₂W_(t-2)+a₃W_(t-3)MOD 6, where a₁, a₂, and a₃ are pre-selected constants.
 6. An electricaldevice according to claim 2 or 3, characterized in that if the number ofsaid plurality of possible step patterns is not a prime number, thensaid pattern select signal (W_(t)) is derived on the basis of saidcombined value (U_(t)) and said previously derived step pattern selectsignals (W_(t-1)) using a Chinese remaindering technique.
 7. Anelectrical device according to claims 1-6, characterized in that saidplurality of possible step patterns is: (0,0,1,1), (0,1,0,1), (1,0,0,1),(0,1,1,0), (1,0,1,0), (1,1,0,0).
 8. An electrical device according toclaims 1-7, characterized in that said device further comprises functiongenerating means (203) adapted to calculate an output value (Out_(t)) asthe sum of said plurality of sequence values (Z_(t)) MOD
 2. 9. Anelectrical device according to claims 1-8, characterized in that saidplurality of sequence generation means is m-sequence generators.
 10. Anelectrical device according to any one of the previous claims,characterized in that said device is used in a mobile telephone.
 11. Amethod of self clock controlled pseudo random noise (PN) sequencegeneration, comprising the step of: outputting a plurality of sequencevalues (Z_(t)) on the basis of a plurality of clock values (C_(t)),characterized in that said method further comprises the step of:selecting a step pattern, providing said plurality of clock values(C_(t)), from a plurality of possible step patterns on the basis of astep pattern select signal (W_(t)).
 12. A method according to claim 11,characterized in that said step pattern select signal (W_(t)) is derivedon the basis of a combined value (U_(t)) and one or more previouslyderived step pattern select signals (W_(t-1)).
 13. A method according toclaim 12, characterized in that a plurality of step control values(u_(t)) is output, and said combined value (U_(t)) is provided on thebasis of said plurality of step control values (u_(t)) and on the basisof a plurality of prior clock values (C_(t-1)).
 14. A method accordingto claim 12 or 13, characterized in that the number of said plurality ofpossible step patterns is 6, and in that said pattern select signal(W_(t)) is derived as: U_(t)+W_(t-1) MOD
 6. 15. A method according toclaim 12 or 13, characterized in that the number of said plurality ofpossible step patterns is 6, and in that said pattern select signal(W_(t)) is derived as: U_(t)+a₁W_(t-1)+a₂W_(t-2)+a₃W_(t-3) MOD 6, wherea₁, a₂, and a₃ are pre-selected constants.
 16. A method according toclaim 12 or 13, characterized in that said pattern select signal (W_(t))is derived on the basis of said combined value (U_(t)) and saidpreviously derived step pattern select signals (W_(t-1)) using a Chineseremaindering technique, if the number of said plurality of possible steppatterns is not a prime number.
 17. A method according to claims 11-16,characterized in that said plurality of possible step patterns is:(0,0,1,1), (0,1,0,1), (1,0,0,1), (0,1,1,0), (1,0,1,0), (1,1,0,0).
 18. Amethod according to claims 11-17, characterized in that said methodfurther comprises the step of calculating a value (Out_(t)) as the sumof said plurality of sequence values (Z_(t)) MOD
 2. 19. A methodaccording to claims 11-18, characterized in that said plurality ofsequence values (Z_(t)) is generated by a plurality of m-sequencegenerators.
 20. A method according to claims 11-19, characterized inthat said method is used in a mobile telephone.